Crystal glass has been used for high quality tablewares and craftwork because of its characteristic properties, such as high transparence and brightness, weight density, beautiful acoustics, and ease of forming and working.
For labeling an article as crystal glass, it must satisfy the requirements that its oxide composition contains, singly or in combination, 10% by weight of ZnO, BaO, PbO, or K2O, and that it has refractive index nd≧1.520 and density ≧2.45 g/cm3 (EC specifications).
Traditional lead (contained as PbO) crystal glass articles and barium (contained as BaO) crystal glass articles have problems in environmental issues and in alkali resistance, because their materials have toxicity or the product glass articles are susceptible to surface erosion upon washing with an alkali.
Common crystal glass articles including known potassium (contained as K2O) crystal glass articles are each not sufficiently suitable for chemical strengthening. Among them, those having small thicknesses are insufficient in strength, because it is difficult to physical strengthen such glass articles having small thicknesses.
To solve these problems, the present inventors have recently developed and prepared for the market a lead-free, barium-free crystal glass having a potassium-zinc (contained as K2O and ZnO) crystal glass composition, which is superior in chemical strengthening suitability in which sodium ions in a surface layer of the formed article are replaced with potassium ions. The crystal glass composition and the chemical strengthened article are disclosed in Japanese Patent No. 3961560.
However, the disclosed composition becomes difficult to be employed particularly in crystal glass tablewares that are formed by machine and produced in mass production, because the disclosed composition contains 6 percent by weight or more of ZnO, and zinc occupies more than 50% of the material cost of the glass due to the recent price rise of zinc.
In addition, the disclosed crystal glass is designed to have a temperature corresponding to the molten glass viscosity (high-temperature viscosity) Log η=2 of 1430° C. or lower so as to enable hand forming via melting in a crucible, whereas it is designed to have a cooling time of 110 to 115 seconds so as to enable gradual or slow forming, in which the cooling time is defined based on the temperature dependency of viscosity. However, for use in tablewares that will be formed by machine after melting in a tank, a demand has been made on glass to have a temperature corresponding to the molten glass viscosity (high-temperature viscosity) Log η=2 of 1450° C. to 1470° C. and a cooling time of about 110 seconds as properties preferred for such high-speed forming.
Patent Document 1: Japanese Patent No. 3961560
When BaO and PbO are excluded from the viewpoint of environment and the content of ZnO is controlled to less than 2 percent by weight from the viewpoint of cost in a crystal glass composition, the crystal glass composition becomes a potassium-zinc crystal glass composition, but this composition can also be classified as a kind of potassium crystal glass composition.
However, no chemical strengthened potassium crystal glass composition has been known, because known potassium crystal glass compositions are not effectively chemical strengthened by a general chemical strengthening process in which, after applying an aqueous potassium solution to a surface of the crystal glass, the crystal glass is subjected to a heat treatment in a tunnel furnace for about 90 minutes so as to replace sodium ions with potassium ions.
While many high quality crystal glass tablewares have small thicknesses (rim thickness: less than 1.2 mm), it is technologically difficult to physical strengthen such thin-wall crystal glass tablewares effectively by quenching portions having small thicknesses to capture tensile stress inside and to impart a compressive stress layer only to a surface thereof.
Specifically, known high quality potassium crystal glass tablewares have underwent neither chemical strengthening nor physical strengthening, and thereby have insufficient mechanical strength and should be always handled with care.
The present inventors revealed in Japanese Patent No. 3961560 that the potassium crystal glass is not suitable for chemical strengthening because: the content of Na2O, which acts as a sodium ion source in ion replacement, is as small as less than 10 percent by weight; and the content of CaO, which impedes the ion replacement, is as large as more than 5 percent by weight. In the invention disclosed therein, not the potassium crystal glass but a potassium-zinc crystal glass is controlled to be suitable for chemical strengthening. This potassium-zinc crystal glass has a composition containing 10 percent by weight or more of Na2O, 4.2 percent by weight or less of CaO, and 6 percent by weight or more of ZnO.
On the other hand, it is apparent that, if such a potassium crystal glass composition containing zinc, which contributes to improvement in chemical durability, in a content of 2 percent by weight or less is rendered to contain Na2O in a content of 10 percent by weight or more, the resulting crystal glass loses its chemical durability, because the glass contains 8 percent by weight or more of K2O belonging to the same category, i.e., alkali oxides, thereby undergoes alkali excess, and this causes a network structure formed by SiO2 to be opened.
All known potassium crystal glass compositions contain more than 5 percent by weight of CaO, but it has not yet been revealed how the chemical strengthening suitability varies when CaO is partially replaced not with ZnO but with another alkaline earth metal oxide (MgO, SrO).
Accordingly, demands have been made to develop a glass composition that is suitable for chemical strengthening and is inexpensive, among potassium crystal glass compositions containing less than 2 percent by weight of ZnO from the viewpoint of cost and containing less than 10 percent by weight of Na2O from the viewpoint of chemical durability.
As used herein a “glass composition suitable for chemical strengthening” refers to such a glass composition that has a sufficiently high ion-replacement rate at a temperature in a heat treatment for a relatively short time, can thereby have a deeper ion-replaced layer, i.e., a compressive stress layer, and can have a higher compressive stress due to its lower stress relaxation.
To verify that chemical strengthening is conducted effectively, the resulting surface compressive stress layer should have a stress and a thickness greater than certain levels set according to the use.
The potassium-zinc crystal glass composition disclosed in Japanese Patent No. 3961560 is imparted with a chemical strengthened compressive stress layer having a stress of more than 1000 kg/cm2 and a thickness of more than 20 μm so as to increase its practical strength, as a result of heat treatment for 90 minutes according to a chemical strengthening process using an aqueous solution.